Oxide coated cathode supports



nited States Patent OXIDE COATED CATHODE SUPPORTS Harry Huber and NguyenThien-Chi, Paris, France, as-

signors to Compagnie Generale de Telegraphie Sans Fil, a corporation ofFrance No Drawing. Application June 14, 1955 Serial No. 515,527

Claims priority, application France June 18, 1954 A 6 Claims. c1.29-1823) strontium and 50% of barium. i

2,924,874 Patented Feb. 16,1960

two hours and the cathode support is shaped according to the usualpractice. A cathode is then made up by coating the support with amixture, for: instance, "-in molecular'proportions, of 5% of calcium,45% of Applicant has found that such a cathode still retained 5 about25% of itsemissive power after having operated with an alkaline earthlayer, for example, a layer of a mixture of barium carbonate, strontiumand calcium, and by sintering the coated support at a high temperature.The nickel support used contains substances which are reducers withrespect to the oxide, and the support is generally obtained by meltingthe nickel and reducer powders together.

The reducing agents incorporated into the nickel support usually aresilicon, aluminum and magnesium or, occasionally, titanium, tantalum ortungsten in metallic form.

Cathodes obtained in this way have comparatively short lift, theiremissive power falling to a hundredth of its initial value, for example,after 10,000 hours operation at 850 C.

It is an object of the invention to provide a cathode having a longeruseful life than the cathodes of the prior art. More particularly, theinvention has for its object to provide an improved compound for themanufacture of oxide coated cathode supports. With these and otherobjects in view, the compound of the invention comprises nickel with anaddition of from 0.1% to 10% by weight of at least one metal carbide,preferably selected from among the carbides of the following metals: W,Mo, Cr, Ta, Nb, V, Hf, Zr, Ti, La, Y, Co, Pr, Th, U, Si, Al. (Thecarbides of said refractory metals being particularly refractory.)

According to a preferred embodiment the compound according to theinvention consists of nickel with an addition of tungsten carbide.

A compound which is particularly suitable for high output cathodesintended to be operated at high temperatures has a carbide content offrom 1 to 4%, while the compound for low output cathodes operating atlow temperatures will preferably have a carbide content carrying from0.1 to 1%.

The invention will be better understood by reference to the followingspecification given by way of illustration only.

Nickel powder, known under the trademark Mond, in grains of 4 to 5 andpowdered tungsten carbide are mixed in the proportion of 3 /2 gms. ofcarbide to 100 gms. of nickel. The carbide used may, for example, be acarbide known under the trademark Murex in grains of about 1 gm., havinga maximum content of 6.12% by Weight of carbon; no more than 0.1% of thecarbon contained in said carbide is in the free state. Other specificexamples by which the present invention may be realized result from thesubstituting for the tungsten carbide any one of the 16 other carbidesof the group tablet is sintered at the temperature of 1250 C., duringnated.

10,000 hours. i

This'increase of the useful life of the'cathode can be explained asfollows: i

It is usually considered that a requisite for a satisfactory operationof an oxide coated cathode is the permanent presence, in thealkaline-earth emissive compound forming the coating of the nickelsupport, of a certain proportion of barium. Now, during the operation,the barium content of the cathode gradually decreases. This loss of thebarium content must be compe'nsated as nearly as possible by the bariumoriginating from the reduction, by the reducing agents contained in 1the nickel support, of the barium oxide contained in the emissivecompound.' This reduction takes place in the solid state by reactiononjthe contact surface between the compound and the support. With themetal reducers generally used, the reduction is usually fairly swift andthe barium which is freed in quantities greater than necessary to offsetthe losses forms an intermediatelevel between the support and theemissive compound. 'These intermediate levels decrease the adherence oftheeim'ssive compound to the cathode support. This, in turn, tends todecrease the reduction of the barium oxide by the reducing agent, whichmakes irregular the liberation of barium.

By using, in accordance with the teaching of the invention, metalcarbides as reducer, an adequate reduction rate 18 achieved and theabove disadvantage is elimivery refractory, they do not emit any vaporslikely to damage the cathode. On the other hand, it is believed that.the products resulting from the reaction of these carbides with theemissive compound and, more particularly, the carbide and the basicbarium tungstate resulting from the reaction of tungsten carbide withthis compound, react in their turn with the reducing carbide and withBaO, thus further providing free barium. This action increases theuseful life of the cathode.

It is to be understood that the invention is not limited to theparticular example described above. Other carbide proportions, grainsizes and carbide mixing methods could be used. Thus, for instance, itis possible to use a carbide powder having larger grains, having a sizeup to 30p and a very low carbide content, when low output cathodes,intended for operating at low temperatures,

are manufactured. Further, carbide powder may be addedto melted nickelin a vacuum oven What we claim is: l

1. A cathode for electron tubes, comprising an 'emis- I sive body and asupport for said body, said support 3. A cathode for electron tubes,comprising an emis sive body and a support for said body, said supportconsisting essentially of nickel containing from 1 to 4% V of its Weightof tungsten carbide.

4. A cathode for electron tubes, comprising an emissive body and asupport for said body, said support On the one hand, as the carbidesused are.

4 consisting of W, Mo, Cr, Ta, Nb, V, Hf, Zr, Ti, La, Y, Co, Pr, Th, U,Si, Al.

References Cited in the file of this patent 5 UNITED STATES-PATENTS1,819,927 Taylor Aug. 18, 1931 2,313,227 De Bats Mar. 9, 1943

1. A CATHODE FOR ELECTRON TUBES, COMPRISING AN EMISSIVE BODY AND ASUPPORT FOR SAID BODY, SAID SUPPORT CONSISTING ESSENTIALLY OF NICKELCONTAINING FROM 0.1 TO 10% OF ITS WEIGHT OF AT LEAST ONE REFRACTORYMETAL CARBIDE.